Regional effects of repetition time on NMR quantitation of water in normal and edematous lungs

It is well known that pulmonary edema is, in general, spatially nonuniform. Since the NMR spin-lattice relaxation time (T1) is increased by lung edema, the spatial distribution of T1 will be nonuniform. When the repetition time (TR) is short relative to the T1 of edematous lung, lung water content will be underestimated and this underestimation will be spatially nonuniform as well. Therefore, technical artifacts which are a complex function of lung edema and its spatial distribution are expected. We compared overall and regional (topographic) lung water density measurements obtained from living rats (with normal or edematous lungs) using repetition times of 2.0 and 6.2 s (at a magnetic field of 1 T), to quantify this uneven T1 effect for normal and edematous lungs. NMR measurements at TR = 2.0 s underestimated whole lung water density (ρ―H2O) (at TR = 6.2 s) by an average of 7.2% in normal rats and 22.5% in rats with pulmonary edema. Regional ρ―H2O underestimation (%Δρ―H2O) varied from 2.2 to 8.8% (group means) in normal lungs and from 7.3 to 30.8% in edematous lungs. As a result, the interquartile range (of the voxel distribution as a function of pH2O) underestimated the spatial nonuniformity of lung water density by 28.0% in edematous lungs, likely because of greater loss of NMR signal from high-water-density, long-T1 lung regions. Both %ΔpH2O and T1, were significantly correlated with ρ―H2O at TR = 6.2 s. Artifactual distortion of the spatial distribution of NMR signal intensity cannot be predicted from average T1 values characterizing the whole lung, but rather requires a knowledge of the spatial distribution of T1. © 1989 Academic Press, Inc.